In writing an essay about the archetypal dystopian characters in the book Uglies, what could be a good conclusion that ties the characters to a global lesson?
Without knowing your thesis, this is a tricky question to answer. I would say, though, that the best concluding paragraphs answer the larger "so what" kind of questions; they connect what you have been saying in your body paragraphs to the larger world. You seem to be on the same page with this when you say you are looking for a lesson that you can globalize.
My suggestion would be to explore some of the prominent themes of the novel and find one that matches the points you are trying to make in your essay. If you are doing the essay on archetypal characters, it seems you're looking at how many dystopian books deal with similar sets of problems and similar characters working through those problems. Consider why we tell the same basic story again and again in this way. How does the dystopian genre capture our fears about the modern world and its problems? Then carry that knowledge over the themes of the book, as these give readers insight into the nature of these problems and how they can be combated. I linked to some of eNote's discussion on themes, but I think one of the strongest ones that might help you is the dangers of conformity and perfection. If you tie in how each of your characters deals with the pressure to be Pretty, you can then wrap up your essay with what their struggles suggest for how readers can deal with similar pressures in real life.